There has heretofore been developed a photoelectric type displacement detecting instrument comprising:
a main optical lattice formed on a long member made of a light transmitting material;
an auxiliary optical lattice formed on a second member made of a light transmitting material and movable relative to the first member in a manner to be parallel and adjacent to the main optical lattice;
a light emitter for emitting detecting light to the both optical lattices in a direction from the outer side of the second member to the first member;
a light receiver opposed to the light emitter, interposing therebetween the both optical lattices, for receiving the detecting light transmitted through the both optical lattices, transducing changes in the value of the received light through the repeat of overlappings due to relative movements between the both optical lattices into electrical signals, and outputting the same; and
an electronic circuit for processing the electric signals from the light receiver and calculating a relative moving distance between the main optical lattice and the auxiliary optical lattice on the basis of the number of chanes in the value of the received light.
The photoelectric type displacement detecting instrument of the type described has been mounted on machine tools, measuring machines or the like for use, for example. As a matter of course, there has been demand for rendering the displacement detecting instrument compact in size and light in weight.
To meet the demand of rendering the instrument compact in size and light in weight, the present applicant has proposed scales for photoelectric type encoders and photoelectric type encoders as disclosed in Japnese Patent Kokai (Post Exam Publn) Nos, 32125/1985, 32126/1985 and 14287/1985.
These are so-called reflective type photoelectric displacement detecting instruments wherein light transmitted through an index scale is reflected by a main scale.
Accordingly, there has been the problem of that rendering the instrument compact in size and integral forming are still unsatisfactory with the so-called transmission type photoelectric displacement detecting instruments wherein light is transmitted through both the main and auxiliary optical lattices.
As shown in FIG. 22, the light emitter in the compact type encoder as being the above-described photoelectric type displacement detecting instrument has been of such an arrangement that light emitted from a light emitting diode 2 is reflected by a concave spherical surface reflector 3C formed of a concave surface mirror-shaped reflective film to illuminate optical lattices 7A and 8A provided on a first and a second scales 7 and 8, respectively, and reaches a photoelectric transducing element 5.
Now, in the photoelectric type encoder using the above-described concave spherical surface reflector 3C, light emitted from the light emitting diode 2 is diffused in the widthwise direction of the optical lattices 7A and 8A, i.e., the direction of graduations. From this reason, such a problem is presented that rise and fall of the signals obtained from the photoelectric transducing element 5 due to formation of bright and dark portions by the repeat of overlappings of the optical lattices 7A and 8A become unclear.
When a diffusion angle of illuminating light is large as described above, a gap between the optical lattices 7A and 8A in the first scale 7 and the second scale 8, i.e., a lattice interval S cannot be increased.
For example, in the case of an optical lattice having a pitch of 20 .mu.m, the lattice interval S should be made as low as about 10 .mu.m or less and accuracy of a guide mechanism for the relative movement between the first and second scales 7 and 8 should be made fairly high, which disadvantageously raise the manufacturing costs.
Here, if a paraxial focal length of the spherical surface of the concave spherical surface reflector 3C is f, and the width of a light emitting portion of the light emitting diode 2 is d, then the diffusion angle is proportional to d/f.
Here, f is proportional to the radius and the magnitude of the diffusion angle is hardly affected by f.
More specifically, diffusion of the illuminating light is principally attributed to the width of the light emitting portion of the light emitting diode 2.
In contrast thereto, it is conceivable that, in order to increase the value of f, the radius of the sphere of the concave spherical reflector 3C is increased, however, this would present the problem of making the photoelectric type encoder large-size.
Furthermore, it is conceivable that, in order to decrease the value of d, the light emitting portion of the light emitting diode 2 is reduced in size, however, this would present the problem of decreasing the value of light emission of the light emitting diode.
The present invention has been developed to obviate the above-described disadvantages of the prior art and has as its object the provision of a transmission type photoelectric displacement detecting instrument wherein the instrument is rendered compact in size, light in weight, and costs are reduced.
Furthermore, the present invention has been developed to obviate the above-described disadvantages of the prior art and has as its another object the provision of a photoelectric type displacement detecting instrument wherein the instrument is not large-sized, a satisfactory value of light emission of the light emitting diode is maintained and adverse influence by diffusion of the illuminating light is minimized.